This invention relates to an indirectly heated cathode of a gas discharge tube which is used as a light source for various analyses and quantitative measurements.
One example of a gas discharge tube is a deuterium lamp as shown in FIG. 5. The deuterium lamp 1 comprises: a transparent sealed envelope 2; and an anode 3, a cathode 4 and a shield electrode 5 which are provided in the envelope 2. The shield electrode 5 has a small hole 6 serving as an electron converging portion, and a light transmission window 7.
When, in the gas discharge tube thus constructed, the cathode 4 is heated and simultaneously a voltage is applied across the anode 3 and the cathode 4, arc discharge is induced between the anode 3 and the cathode 4 through the small hole 6, thus producing light. Only part of a positive column can pass through the small hole 6, thus producing a spot light which is transmitted through the light transmission window 7.
An indirectly heated cathode for such a deuterium lamp 1 has been disclosed by Japanese Patent Application Examined Publication No. 56628/1987. As shown in FIG. 3, a double coil (coating coil) 9 of a tungsten filament is wound around the outer wall of a heat-resisting and thermally conductive cylinder 8. An electron emitting material layer 10 is formed in such a manner as to contain the double coil 9 by filling the space between the turns of the primary and secondary coils of the double coil 9 with barium carbonate, strontium carbonate or calcium carbonate, or a mixture of them. A coiled heater 11 is inserted into the cylinder 8. The cylinder 8 is conductively connected to the heater 11 through a support 12, and installed in the discharge tube. The discharge tube thus fabricated is evacuated to 10.sup.-3 Torr or less, and current is applied to the heater 11. As a result, the above-described carbonates are thermally decomposed, and the electron emitting material layer 10 of oxides is completed.
A conventional indirectly heated cathode needs a larger quantity of heat when preheated and operated: W.sub.pr =6.37 W when preheated (where W.sub.pr is a quantity of heat required for the cathode to start discharging, or a quantity of heat required for the cylinder surface temperature to reach 700.degree. C.), W.sub.ou =2.4 W when operated (where W.sub.ou is a quantity of heat which the heater applies to the cathode during discharging, being called "forced heating"); that is, W.sub.ou /W.sub.pr =0.38. Thus, the cathode is different in specification from a conventional directly heated cathode as follows:
______________________________________ Conventional indirectly heated Directly heated cathode cathode ______________________________________ Preheating voltage 10 V 10 V Preheating current 1.1 A 0.8 A Operating voltage 7 V 3.5 V Operating current 0.8 A 0.3 A ______________________________________
As is apparent from the above-described table, the preheating current and the operating voltage of the conventional indirectly heated cathode are larger than those of the directly heated cathode. Therefore, the indirectly heated cathode type gas discharge tube is not interchangeable with the corresponding (10 V) directly heated cathode type gas discharge tube.